Certain motors, generators, and other rotating electrical machinery conventionally include commutators or analogous rotor contact surfaces. These machines additionally utilize low-resistance brushes, typically positioned in holders, to transfer electric current between the commutators and stationary components of the devices. Because the brushes of a machine frictionally contact its rotating commutator, wear is likely to occur. To avoid (or at least reduce) erosion of the commutator--which often is constructed of copper bars--corresponding brushes are made of softer materials, typically carbon and graphite. As a consequence, conventional brushes deteriorate in use.
Rather than protecting the commutator surface, however, excessive brush wear may damage it. If substantially or completely eroded, the relatively soft carbon and graphite of the brush no longer is available to engage the commutator surface. Instead, the surface of the commutator may contact the copper shunt wire typically connected to the brush or some portion of the brush holder itself. Either contact is likely to be sufficiently detrimental to continued operation of the commutator that premature replacement of the brush may be desirable.
Notwithstanding the potential desirability for this reason of replacing brushes before they wear substantially, disadvantages of doing so exist. In particular, prematurely replacing brushes obviously increases over time the cost of materials, as more brushes than optimal must be used to produce the same electrical output. Additionally, stopping rotation of the machinery to replace brushes ceases output entirely.
Monitoring systems were thus created to address these brush-wear issues. Exemplary existing systems are illustrated in FIGS. 1-4 of U.S. Pat. No. 4,316,186 to Purdy, et al., which patent is incorporated herein in its entirety by this reference. Each system includes a sensing wire embedded longitudinally in part of its corresponding brush. Both the periphery and the tip of the sensing wire are electrically insulated from the brush, thereby isolating the wire electrically from current flowing through the brush. The tip, moreover, is remote from the edge of the brush that initially contacts the commutator surface.
As the brush of the Purdy patent wears in use, the commutator surface eventually contacts the insulated tip of the sensing wire. Continued use of the brush causes the insulation to erode from the tip of the wire, exposing the wire to the surface of the commutator. Upon this exposure, the voltage present on the commutator surface also becomes present on the sensing wire, providing an electrical signal indicating that the contact has occurred. Because the sensing wire extends further through the brush than the current-carrying output leads, it contacts the commutator surface--and thus senses such contact--before the (harder) output leads do so. According to the Purdy patent:
This is to allow the dynamoelectric machine to be operated until the next opportunity for maintenance, and brush replacement, of the dynamoelectric machine, rather than causing a sudden shut down of the dynamoelectric machine, which is not desirable in most applications for dynamoelectric machines.
See col. 5, lines 56-62.
German Patentschrift No. 4137384 appears to disclose another existing wear-indicating system. When the sensing wire of the system disclosed in the German document contacts the commutator and has its insulation removed frictionally, it begins conducting electricity. The conducted electricity illuminates a warning light, effectively indicating that further use of the brush might damage the commutator. Other uses of the conducted electrically may be made as well.
In each of the above-referenced systems, the sensing wire functions as a discrete, two-state device: if the brush has worn to a predetermined level sufficient to cause the sensing wire to contact the rotating surface, the wire conducts electricity; if the brush has not, the sensing wire does not conduct. No continuous or incremental information respecting brush erosion is thus supplied by these systems. None, moreover, necessarily operates reliably under conditions where the sensing wire is stressed. If the sensing wire breaks under such stress, for example, it will fail to provide the requisite signal indicating that the insulation at its tip has been worn away. Similarly, if the sensing wire separates from the brush, no wear signal will be supplied.